1. Field of the Invention
The present invention generally relates to catalysts and processes for the catalytic partial oxidation of light hydrocarbons (e.g., methane or natural gas) using a supported rhodium-spinel catalyst to produce a mixture of carbon monoxide and hydrogen.
2. Description of Related Art
Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive. To improve the economics of natural gas use, much research has focused on methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is reformed with water to produce carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted to hydrocarbons, for example, using the Fischer-Tropsch process to provide fuels that boil in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes.
Current industrial use of methane as a chemical feedstock proceeds by the initial conversion of methane to carbon monoxide and hydrogen by either steam reforming, which is the most widespread process, or by dry reforming. Steam reforming currently is the major process used commercially for the conversion of methane to synthesis gas, proceeding according to Equation 1.CH4+H2O⇄CO+3H2  (1)
Although steam reforming has been practiced for over five decades, efforts to improve the energy efficiency and reduce the capital investment required for this technology continue.
The catalytic partial oxidation of hydrocarbons, e.g., natural gas or methane to syngas is also a process known in the art. While currently limited as an industrial process, partial oxidation has recently attracted much attention due to significant inherent advantages, such as the fact that significant heat is released during the process, in contrast to steam reforming processes.
In catalytic partial oxidation, natural gas is mixed with air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperature and pressure. The partial oxidation of methane yields a syngas mixture with a H2:CO ratio of 2:1, as shown in Equation 2.CH4+1/2O2→CO+2H2  (2)
This ratio is more useful than the H2:CO ratio from steam reforming for the downstream conversion of the syngas to chemicals such as methanol and to fuels. The partial oxidation is also exothermic, while the steam reforming reaction is strongly endothermic. Furthermore, oxidation reactions are typically much faster than reforming reactions. This allows the use of much smaller reactors for catalytic partial oxidation processes. The syngas in turn may be converted to hydrocarbon products, for example, fuels boiling in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes by processes such as the Fischer-Tropsch synthesis.
The selectivities of catalytic partial oxidation to the desired products, carbon monoxide and hydrogen, are controlled by several factors, but one of the most important of these factors is the choice of catalyst composition. Difficulties have arisen in the prior art in making such a choice economical. Typically, catalyst compositions have included precious metals and/or rare earths. The large volumes of expensive catalysts needed by prior art catalytic partial oxidation processes have placed these processes generally outside the limits of economic justification.
For successful operation at commercial scale, the catalytic partial oxidation process must be able to achieve a high conversion of the methane feedstock at high gas hourly space velocities, and the selectivity of the process to the desired products of carbon monoxide and hydrogen must be high. Such high conversion and selectivity must be achieved without detrimental effects to the catalyst, such as the formation of carbon deposits (“coke”) on the catalyst, which severely reduces catalyst performance. Accordingly, substantial effort has been devoted in the art to the development of catalysts allowing commercial performance without coke formation.
An attempt at synthesis gas production by catalytic partial oxidation to overcome some of the disadvantages and costs typical of steam reforming is described in European Patent No. 303,438, entitled “Production of Methanol from Hydrocarbonaceous Feedstock.” Certain high surface area monoliths of cordierite (MgO/Al2O3/SiO2), Mn/MgO cordierite (Mn—MgO/Al2O3/SiO2), mullite (Al2O3/SiO2), mullite aluminum titanate (Al2O3/SiO2—(Al,Fe)2O3/TiO2), zirconia spinel (ZrO2/MgO/Al2O3), spinel (MgO/Al2O3), alumina (Al2O3) and high nickel alloys are suggested as catalysts for the process. The monoliths may be coated with metals or metal oxides that have activity as oxidation catalysts, e.g., Pd, Pt, Rh, Ir, Os, Ru, Ni, Cr, Co, Ce, La, and mixtures thereof. Other suggested coating metals are noble metals and metals of groups IA, IIA, III, IV, VB, VIB, or VIIB of the periodic table of the elements.
A number of process regimes have been proposed for the production of syngas via catalyzed partial oxidation reactions. For example, the process described in U.S. Pat. No. 4,877,550 employs a syngas generation process using a fluidized reaction zone. Such a process however, requires downstream separation equipment to recover entrained supported-nickel catalyst particles. To overcome the relatively high pressure drop associated with gas flow through a fixed bed of catalyst particles, which can prevent operation at the high gas space velocities required, various structures for supporting the active catalyst in the reaction zone have been proposed. U.S. Pat. No. 5,510,056 discloses a monolithic support such as a ceramic foam or fixed catalyst bed having a specified tortuosity and number of interstitial pores that is said to allow operation at high gas space velocity. Catalysts used in that process include ruthenium, rhodium, palladium, osmium, iridium, and platinum. Data are presented for a ceramic foam supported rhodium catalyst at a rhodium loading of from 0.5-5.0 wt %.
U.S. Pat No. 5,648,582 also discloses a process for the catalytic partial oxidation of a feed gas mixture consisting essentially of methane. The methane-containing feed gas mixture and an oxygen-containing gas are passed over an alumina foam supported metal catalyst at space velocities of 120,000 hr.−1 to 12,000,000 hr.−1 The catalytic metals exemplified are rhodium and platinum, at a loading of about 10 wt %.
Certain catalysts containing Group VIII metals such as nickel or rhodium on a variety of supports have been described. For example, V. R. Choudhary et al. (“Oxidative Conversion of Methane to Syngas over Nickel Supported on Low Surface Area Catalyst Porous Carriers Precoated with Alkaline and Rare Earth Oxides,” ((1997) J. Catal., 172: 281-293) disclose the partial oxidation of methane to syngas at contact times of 4.8 ms (at STP) over supported nickel catalysts at 700 and 800° C. The catalysts were prepared by depositing NiO—MgO on different commercial low surface area porous catalyst carriers consisting of refractory compounds such as SiO2, Al2O3, SiC, ZrO2 and HfO2. The catalysts were also prepared by depositing NiO on the catalyst carriers with different alkaline and rare earth oxides such as MgO, CaO, SrO, BaO, Sm2O3 and Yb2O3.
U.S. Pat. No. 4,690,777 also discloses catalysts comprising Group VIII metals, such as Ni, on porous supports, for use in reforming hydrocarbons to produce CO and H2. U.S. Pat. No. 5,500,149 discloses various transition metals that can act as catalysts in the reaction CO2+CH4→2CO+2H2, and demonstrates how reaction conditions can affect the product yield.
U.S. Pat. No. 5,149,464 discloses a method for selectively converting methane to syngas at 650° C. to 950° C. by contacting the methane/oxygen mixture with a solid catalyst comprising a supported d-Block transition metal, transition metal oxide, or a compound of the formula MxM′yOz wherein M′ is a d-Block transition metal and M is Mg, B, Al, Ga, Si, Ti, Zr, Hf or a lanthanide.
The partial oxidation of methane to synthesis gas using various transition metal catalysts under a range of conditions has been described by Vernon, D. F. et al. ((1990) Catalysis Letters 6:181-186). European Pat. App. Pub. No. 640561 discloses a catalyst for the catalytic partial oxidation of hydrocarbons comprising a Group VIII metal on a refractory oxide having at least two cations.
U.S. Pat. No. 5,447,705 discloses an oxidation catalyst having a perovskite crystalline structure and the general composition: LnxA1-yByO3, wherein Ln is a lanthanide and A and B are different metals chosen from Group IVb, Vb, VIb, VIIb or VIII of the Periodic Table of the Elements. The catalyst is said to have activity for the partial oxidation of methane.
U.S. Pat. No. 5,105,044 discloses a process for synthesizing hydrocarbons having at least two carbon atoms by contacting a mixture of methane and oxygen with a spinel oxide catalyst of the formula AB2O4, where A is Li, Mg, Na, Ca, V, Mo, Mn, Fe, Co, Ni, Cu, Zn, Ge, Cd or Sn and B is Na, Al, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, Ge, Rh, Ag or In, A and B being different elements.
U.S. Pat. No. 5,653,774 discloses a spinel catalyst of the formula M2+M23+O4 where M2+ is at least one member of a group consisting of Mg2+, Zn 2+, Ni2+, Fe2+, Cu2+, Co2+, Mn2+, Pd2+ and Pt2+, and M3+ is at least one member of a group consisting of Al3+, B3+, Cr3+, Fe3+, Ga3+, In3+, La3+, Ni3+, Co3+, Mn3+, Rh3+, Ti3+ and V3+ ions, for the preparation of synthesis gas from a hydrocarbyl compound. The catalyst is prepared by heating hydrotalcite-like compositions having the general formula [M2+(1-x)Mx3+(OH2)]x+(Ax/nn-1)·mH2O.
U.S. Pat. No. 5,338,488 describes a process for the catalytic steam reforming of methane or natural gas to synthesis gas. The catalyst employed in that process is NiO supported on calcium aluminate, alumina, spinel type magnesium aluminum oxide or calcium aluminate titanate) and the reaction conditions include elevated temperature (850°-1,000° C.) and pressure (10-40 atm), a gas hourly space velocity of about 5000-8000 per hour at a steam/carbon mole ratio of 2-5.
U.S. Pat. No. 5,025,109 describes spinel oxide catalysts such as ZnMn2O4 that are active for catalyzing the direct partial oxidation of methane with oxygen to produce hydrocarbons having at least two carbon atoms. U.S. Pat. No. 5,238,898 describes a process for upgrading methane to higher hydrocarbons using spinel oxide catalysts such as MgMn2O4 or CaMn2O4, modified with an alkali metal such as Li or Na.
British Pat. No. GB2247465 describes certain catalysts comprising platinum group metals supported on inorganic compounds such as oxides and/or spinels of aluminum, magnesium, zirconium, silicon, cerium and/or lanthanum, and combinations thereof, together with an alkaline metal in some cases. These catalysts are said to be active for producing synthesis gas from methane by means of reforming and combustion reactions, optionally in the presence of steam.
PCT Patent Application Publication No. WO 01/12540 describes steam reforming of a hydrocarbon over certain spinel-supported rhodium catalysts. Suitable hydrocarbon feeds for that process are said to be oxygenates, alkanes, alkenes, alkynes, branched isomers, aromatics, saturated and unsaturated hydrocarbons and combinations thereof, including fuels such as gasoline, kerosene, diesel and JP-8.
One disadvantage of many of the existing catalytic hydrocarbon conversion methods is the need to include steam in the feed mixture to suppress coke formation on the catalyst. Another drawback of some of the existing processes is that the catalysts that are employed often result in the production of significant quantities of carbon dioxide, steam, and C2+ hydrocarbons. Also, large volumes of catalyst are sometimes required, necessitating the use of exceptional devices in an attempt to evenly distribute the feed to the top of the catalyst bed. None of the existing processes or catalysts are capable of providing high conversion of reactant gas and high selectivity of CO and H2 reaction products. Accordingly, there is a continuing need for a process and catalyst for the catalytic partial oxidation of hydrocarbons, particularly methane, or methane containing feeds, in which the catalyst retains a higher level of activity and selectivity to carbon monoxide and hydrogen under conditions of high gas space velocity, elevated pressure and high temperature.